In recent years, as described in Appl. Phys. Lett., Vol. 67, Issue 21, pages 3114-3116 (1995) by Stephan Y. Chou et al., a fine processing technology, for pressure-transferring a fine structure provided on a mold onto a work (or workpiece), such as a semiconductor, glass, resin, or metal, has been developed and has received attention. This technology is called “nanoimprint” or “nanoembossing” since it has a resolving power on the order of several nanometers. In addition to semiconductor manufacturing, the technology is capable of effecting simultaneous processing of a three-dimensional structure at a wafer level. For this reason, the technology has been expected to be applied to a wide variety of fields, such as manufacturing technologies, and the like, for optical devices, such as a photonic crystal, and the like, μ-TAS (micro total analysis systems), biochips, etc.
The case in which such nanoimprint, e.g., an optical imprint method, is used in the semiconductor fabrication will be described below.
First, on a substrate (e.g., a semiconductor wafer), a resin layer of a photocurable resin material is formed. Next, against the resin layer, a mold, on which a desired imprint pattern is formed, is pressed, followed by irradiation with ultraviolet rays, to cure the photocurable resin material. As a result, the imprint structure is transferred onto the resin layer. Then, etching, or the like, with the resin layer as a mask is effected to form a desired structure on the substrate.
Incidentally, in the semiconductor manufacturing, it is necessary to effect (positional) alignment of the mold with the substrate. For example, in such a current circumstance that a semiconductor process rule is not more than 100 nm, a tolerance of an alignment error due to an apparatus is such a severe degree that it is said to be several nanometers to several tens of nanometers.
As such an alignment method, e.g., U.S. Pat. No. 6,696,220 has proposed a method in which a mold and a substrate are caused to contact each other, in a state in which a resinous material is interposed therebetween, to effect alignment. In this method, first, a photocurable resin material is selectively applied to a portion of the substrate other than an alignment mark provided to the substrate. Next, the substrate is moved to a position opposite to the mold. In this state, a distance between the mold and a work (the substrate provided with the photocurable material) is decreased, so that the mold is caused to come near to such a distance that the alignment mark is not filled with the resin material. In the method, the alignment is effected in this state and thereafter, final pressure application is performed. In the method, an optical system for alignment employs such an observation method that only a portion having a small depth of focus in the neighborhood of the alignment mark on the mold side is observed.
More specifically, the marks provided on the mold and the substrate, respectively, are formed into an image in a single image pickup device by utilizing chromatic aberration.
However, references of the mold and the substrate are actually different from each other in many cases. In a case in which a mark provided to a mold formed of quartz having a high transmittance, and a mark provided to a substrate formed of silicon having a high reflectance, overlap each other, when the marks are projected in a perpendicular direction, the mold-side mark in a pickup image can be optically blocked by the substrate-side mark.
In such a state, images of the alignment marks provided to the mold and the substrate, respectively, cannot be picked up with a sufficient gradation range, so that a high detection resolution cannot be attained in some cases.